The present invention relates to a process for preparing coating compositions containing microcapsules. In particular it relates to a process for concentrating an aqueous slurry of microcapsules to provide a high solid ink which can be press applied with little or no drying.
In the manufacture of pressure-sensitive recording papers, a layer of pressure-rupturable microcapsules containing a solution of colorless dyestuff precursor is coated on the back side of the front sheet of paper of a carbonless copy paper set. This coated backside is known as the CB coating. In order to develop an image or copy, the CB coating is mated with a paper containing a coating of a suitable color developer, also known as dyestuff acceptor, on its front. This coated front color developer coating is called the CF coating. The color developer is a material, usually acidic, capable of forming the color of the dyestuff by reaction with the dyestuff precursor.
Marking of the pressure-sensitive recording papers is effected by rupturing the capsules in the CB coating by means of pressure to cause the dyestuff precursor solution to be exuded onto the front of the mated sheet below it. The colorless or slightly colored dyestuff, or dyestuff precursor, then reacts with the color developer in the areas at which pressure was applied, thereby effecting the colored marking. Such mechanism for the technique of producing pressure-sensitive recording papers is well known.
Among the well known color developers used on CF record sheets are phenolic-type resins, such as acetylated phenolic resins, salicylic acid modified phenolics and, particularly, novolac type phenolic resins.
Among the well known basic, reactive, colorless chromogenic dye precursors useful for developing colored marks when and where applied to a receiving sheet coated with such color developers are Crystal Violet Lactone (CVL), the p-toluenesulfonate salt of Michler's Hydrol or 4,4'-bis(diethyllamino)benzhydrol, Benzoyl Leuco Methylene Blue (BLMB), Indolyl Red, Malachite Green Lactone 8'-methoxybenzoindoline spiropyran, Rhodamine Lactone, and mixtures thereof.
A number of microencapsulation techniques have been used to prepare oil-containing microcapsules. Some of the principal techniques are complex coacervation (typically used to prepare gelatin capsules), in situ polymerization (typically used to prepare polyurethane and polyurea capsules).
For some applications it is desirable to separate the microcapsules from the dispersion in which they are prepared. One such application is the preparation of coating compositions which are designed to be printed on or spot coated on paper to provide a carbonless form.
A number of techniques have been used to separate microcapsules. One of the principal techniques is spray drying. U.S. Pat. No. 4,139,392 to Davis et al. discloses a hot melt coating composition containing microcapsules in which microcapsules are spray dried to form a free flowing powder which is dispersed in a wax composition with the aid of an anionic dispersing agent.
U.S. Pat. No. 4,171,981 to Austin et al. describes another method for preparing a print on composition containing microcapsules in which an aqueous slurry of microcapsules is mixed with a hot melt suspending medium and a wiped film evaporator is used to remove the water.
U.S. Pat. No. 4,729,792 to Seitz discloses yet another method in which microcapsules are prepared by interfacial crosslinking of a polysalt formed by reaction of a polyamine and a polyanionic emulsifier with a polyisocyanate. The microcapsules are separated by adding a lipophilizing agent to the capsule slurry. The lipophilizing agent reacts with the polyanionic emulsifier and renders it non-polar such that the microcapsules precipitate from the slurry. The microcapsules can then be dispersed in an ink vehicle with the aid of a dispersing agent. It should be noted that dispersing agents are necessary for dispersing in both polar and non-polar printing ink vehicle.